1. Field of the Invention
This invention relates to metalorganic chemical vapor deposition (MOCVD) reactors and more particularly to susceptors used in MOCVD reactors.
2. Description of the Related Art
Growth of gallium nitride (GaN) based semiconductor devices in MOCVD reactors is generally described in DenBaars and Keller, Semiconductors and Semimetals, Vol. 50, Academic Press Inc., 1997, p. 11-35. MOCVD is a nonequilibrium growth technique that relies on vapor transport of the precursers and subsequent reactions of group III alkyls and group V hydrides in a heated zone. Growth gasses and dopants are supplied to the reactor and are deposited as epitaxial layers on a substrate or wafer. One or more wafers usually rest on a structure of graphite called a susceptor that can be heated by a radio frequency (RF) coil, resistance heated, or radiantly heated by a strip lamp or coil heater. During the growth process, the heated susceptor heats the wafers.
FIG. 1 shows a conventional susceptor 10 that is used in MOCVD reactors such as those provided by Thomas Swan Scientific Equipment Limited. It has a hollowed cylindrical shape and is mounted over the reactor's heating element at the bottom of the reactor, below the source gas inlet. It has a circular base plate 12 and cylindrical sleeve 13, with the circular plate 12 having a series of disk shaped depressions 14 equally spaced around the susceptor's longitudinal axis. Each of the depressions 14 can hold a semiconductor wafer during growth. When the susceptor 10 is heated by the heating element the semiconductor wafers are also heated. When source gases enter the MOCVD reactor, they combine and then deposit on the heated semiconductor wafers as epitaxial layers. The susceptor 10 can typically spin at speeds in the range of 1,000 to 2,000 rpm, which results in more uniform epitaxial layers on the wafers.
Conventional susceptors 10 are usually formed from a monolithic structure of graphite or coated graphite that absorbs heat from the heater element and conducts it to the wafers in contact with the susceptor 10. The entire susceptor 10 is heated uniformly to achieve consistent growth conditions across the surfaces of the wafers. During fabrication of the epitaxial layers, materials will not only deposit on the heated wafer, but will also deposit on the heated susceptor 10. This can cause deposition of significant amounts of GaN, InGaN, AlInGaN, and similar compounds on the susceptor surfaces. The result is a buildup of reaction deposits on the susceptor that can adversely impact subsequent fabrication steps. For instance, the deposits can act as impurities during subsequent growth of the epitaxial layers and can also result in poor interface transition between different layers. For example, if a layer using an indium source gas was grown, indium can be deposited on the susceptor. Though the next layer to be grown does not include indium, indium from the susceptor surfaces can be included in the transition between layers. These impurities can cause poor device performance and can prevent consistent reproduction of semiconductor devices on the wafer.
Another disadvantage of conventional susceptors is that the heating element heats the entire susceptor, not just the areas under or around the wafers. This requires large amounts of heat because the susceptor has a relatively large surface area in comparison to the wafers. Most of the energy is wasted by not heating the wafers. This taxes the heater, contributing to early heater failures. Also, more reactants are consumed due to the fact that the entire susceptor is at a temperature sufficient for chemical vapor deposition.
Another disadvantage of conventional susceptors is that they are difficult to manufacture. They must be machined from a large section of graphite and if any part of the susceptor is damaged the entire structure can be unusable. The fabrication of the depressions can be extremely difficult because they are off set from the structure's longitudinal axis. The depressions cannot be machined using a simple lathe, but must involve more complex processes. For the same reasons it is very difficult to modify the shape of the surface of the depressions to compensate for temperature non-uniformity.